1. Field of the Disclosure
Embodiments disclosed herein relate generally to reduction of benzene content in a reformate stream. More specifically, embodiments disclosed herein relate to reduction of benzene concentrations in a reformate stream via alkylation. More specifically still, embodiments disclosed herein relate to reactive distillation for the alkylation of benzene with an olefin in presence of a heterogeneous slurry catalyst that can be continuously regenerated and replaced during operation.
2. Background
The demand for cleaner and safer transportation fuels is becoming greater every year. Reformate, a product of catalytic reforming, is one of the major sources of feedstock for gasoline blending. However, reformate presents a problem meeting strict environmental and health regulations. For example, light reformate typically contains unacceptably high levels of benzene, a known carcinogen.
Refiners in the U.S. and other countries are required to remove benzene from the reformate stream prior to gasoline blending. Practical options to date include extraction, hydrogenation, transalkylation, and alkylation. Each of these options presents challenges, especially to a small or non-integrated refiner, from both a standpoint of cost and feasibility.
Extraction of benzene requires expensive capital investment in necessary equipment and a customer for the benzene product, neither of which may be feasible for a small non-integrated refiner. Also, while it is possible to extract benzene from the gasoline pool by fractionation techniques, such techniques are not preferred, because the boiling point of benzene is too close to that of some of the more desirable organic components, including C6 paraffins and isoparaffins. Monoalkylate aromatics (monoalkylate), such as toluene and xylenes, are more desirable for gasoline blending, as opposed to benzene, because they are less objectionable both from an environmental and a safety point of view.
Alternatively, benzene in reformate may be removed via hydrogenation. However, hydrogenation of aromatics, such as benzene, toluene, and xylenes, results in reduced octane rating of the reformate stream, and thus diminishes the overall value of the fuel. As with extraction, hydrogenation of benzene also may not feasible for a small refiner due to potentially uneconomical costs associated with supplying hydrogen.
Transalkylation of benzene in reformate with polyalkylate to form monoalkylate product is another option available to refiners. Transalkylation may be feasible when a significant source of polyalkylate is readily available. For example, depending on the catalytic reformer operation, the heavy reformate may contain an adequate amount of polyalkylate to facilitate transalkylation of benzene in the light reformate. Polyalkylate typically is not a desired product of catalytic reforming, because of certain restrictions on its content in gasoline. If the polyalkylate content in reformate is insufficient and there is no alternative polyalkylate source available, transalkylation may not be a feasible option to reduce the benzene content in light reformate. Even if polyalkylate feed is readily available, it may still be more economical to reduce benzene in light reformate via alkylation instead of transalkylation.
As described in U.S. Pat. Nos. 4,371,714 and 4,469,908, alkylation of aromatics, such as benzene, is a more mature and better developed technology than transalkylation. The benzene content in light reformate may be reduced by alkylation with an olefin to produce monoalkylate product. Olefinic feed may be available as certain low-value refinery streams, for example, the fluidized catalytic cracker (FCC) off-gas. If an olefin feed is available, its fuel value and the fuel value of the benzene-containing light reformate stream may be upgraded via alkylation, wherein an olefin combines with benzene to form a high-octane value monoalkylate. A typical benzene transalkylation reaction is shown below:

Older alkylation technology, still widely employed in the petrochemical industry, involves the use of a catalyst based on phosphoric acid. As disclosed in U.S. Pat. No. 5,446,223, alkylation reactions may instead utilize non-polluting, non-corrosive, regenerable materials, such as zeolitic molecular sieve catalysts. U.S. Pat. Nos. 4,371,714 and 4,469,908 disclose straight pass alkylation and transalkylation of aromatic compounds using zeolitic molecular sieve catalysts in fixed beds.
A major problem with alkylation of benzene in reformate using a zeolitic catalyst is rapid deactivation of the catalyst due to coking and poisoning, each of which may result in frequent unplanned unit shut downs or other process interruptions, such as for thermal regeneration of the catalyst. U.S. Pat. No. 5,118,897 further discloses a process for reactivating the zeolitic alkylation catalyst by temporary substitution of the olefin supply stream with a hydrogen stream under certain conditions to shorten the thermal catalyst regeneration cycle.
The catalyst deactivation rate due to coking or poisoning may be reduced by maintaining the zeolitic catalyst in at least a partial liquid phase, such as a hydrocarbon slurry. U.S. Pat. Nos. 5,080,871 and 5,118,872 disclose a moving bed reactor for alkylation and transalkylation of aromatic compounds, in which a slurry is produced by adding solid catalyst to the aromatic feed stream and is circulated through the reactor.
One advantage of a moving bed catalyst slurry reactor, as taught by U.S. Pat. Nos. 5,080,871 and 5,118,872, is that the catalyst may be continuously replaced and regenerated during operation, thus reducing the need for additional unit shut downs. The ability to remove deactivated catalyst on-line eliminates the need to remove catalyst poisons from the feeds or regenerate the catalyst in a fixed bed reactor, thus reducing the cost of the benzene removal unit.
To date, benzene removal by alkylation has not been found economical, because of the high capital equipment cost, including new catalyst regeneration facilities. Therefore, there is still a significant need in the art for improved and cost-efficient methods to reduce benzene in reformate streams, especially for smaller refining operations.